Electric motor having rotor-embedded sensor

ABSTRACT

An electromechanical device comprises a stator and a rotor which is capable of rotating relative to the stator. The rotor has a sensor embedded therein. In a preferred embodiment, the sensor is disposed within a plurality of laminations that are stacked one on top of another. For example, one of the plurality of laminations may be a sensor lamination, in which case the sensor is at least partially disposed within a cavity formed in the sensor lamination. Advantageously, the sensor may be used to directly measure rotor-associated operating conditions, such as rotor current (including the magnitude, frequency and phase of the rotor current), rotor fatigue, rotor temperature, rotor airgap, rotor flux and rotor torque. The sensor is especially well-suited for measuring current in the rotor of an induction motor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to electric motors and, in particular, relates to an electric motor having a rotor-embedded sensor. In a particularly preferred embodiment, the present invention relates to an induction motor having a rotor-embedded sensor that measures the current through one or more rotor windings.

2. Description of Related Art

Electric motors comprise a stator and a rotor that is capable of rotating relative to the stator. For precise motor control, it is generally desirable to know as much as possible regarding motor operating conditions, including conditions that are parameters which can be measured and assigned a value. Consequently, a wide variety of sensors have been provided that are useful for sensing motor operating conditions. For example, position and velocity transducers have been provided that can be mounted to an output shaft of a motor so that the angular position and/or angular velocity of the motor may be measured. As well, current sensors have been provided that can be coupled to the stator windings so that the current flowing the stator windings may be measured.

However, few if any sensors have been provided that are capable of directly measuring conditions associated with the rotor. For example, no known sensor has been provided that is capable of measuring current in the rotor windings of an induction motor. In an induction motor, current is induced in the rotor windings magnetically and, as a result, there are no electrical connections between the rotor windings and the remainder of the motor that can serve as the basis for performing such measurements. Thus, the rotor current cannot be directly measured using existing approaches.

Nevertheless, the ability to perform such measurements would be extremely useful. The magnitude of the current in the rotor windings of an induction motor is indicative of the torque produced by the motor shaft. Additionally, the frequency of the current in the rotor windings is the slip frequency, which can be used to determine the speed of the motor shaft. Finally, knowledge of the phase of the rotor current could be used to synchronize the phase of the current that is applied to the stator windings with the angular position of the rotor. Thus, the ability to measure rotor current would be extremely useful because it would enhance the ability to precisely control the induction motor.

Beyond motor control, the ability to measure rotor current would also be extremely useful for other reasons. For example, the rotor windings of an induction motor are generally formed of solid metal bars that are subject to fatigue and breaking. Currently, broken rotor bars are diagnosed by the acoustical noise and/or the excessive heat that is generated when a rotor bar breaks. However, it is often not possible to diagnose breaking in this way until several rotor bars windings are affected, by which time the motor must be replaced immediately. Therefore, it would be extremely useful to be able to measure the rotor current as a way of detecting broken rotor bars.

Other parameters that are useful to measure include rotor temperature, rotor airgap, rotor flux and rotor torque. Measuring these parameters is useful not only in conjunction with induction motors, but also in conjunction with all types of motors.

Accordingly, what is needed is a way to measure these and other rotor-associated operating conditions.

BRIEF SUMMARY OF THE INVENTION

A preferred embodiment of the present invention provides a way to measure rotor-associated operating conditions, such as rotor current (including the magnitude, frequency and phase of the current), rotor fatigue, rotor temperature, rotor airgap, rotor flux and rotor torque. In general, the present invention may be used with all types of electromechanical devices including both electric motors and electric generators. In an especially preferred embodiment, the present invention provides a way to measure the rotor current in an induction motor.

A preferred embodiment of the present invention provides an electromechanical device that comprises a stator and a rotor which is capable of rotating relative to the stator. The rotor has a sensor embedded therein. In an especially preferred embodiment, the sensor is disposed within a plurality of laminations that are stacked one on top of another. For example, one of the plurality of laminations may be a sensor lamination, in which case the sensor is at least partially disposed within a cavity formed in the sensor lamination.

According to an other preferred aspect, the sensor is a current sensor. For example, the current sensor may comprise a current transformer having a primary winding that is partially formed by a rotor bar that forms part of a rotor winding. Alternatively, a current sensor may be used that has a magnetic core which substantially encircles one of the rotor bars and a Hall effect device which is interposed between first and second adjacent ends of the magnetic core. The current measurement (magnitude, frequency, and/or phase) that is thereby obtained may then be used as a basis for controlling the motor. Alternatively, the sensor may also comprise a plurality of current sensors, each which measures current through a respective rotor bar. In this case, the sensor could be used to sense rotor bar fatigue.

Another preferred embodiment of, the present invention provides a method comprising providing an electric motor having a stator and a rotor that rotates relative to the stator, and sensing a motor operating condition using a sensor. The rotor includes the sensor that is used to sense the motor operating condition. Accordingly, the method also comprises the steps of transmitting information pertaining to the motor operating condition from the rotor to a motor control system. The motor control system is used to control the motor at least partially based on the information pertaining to the motor operating condition. Preferably, during the sensing step, the sensor senses current flowing through a rotor bar that partially forms a rotor winding of the rotor. In this case, the motor control system controls the motor based at least partially on the current measurement that is provided by the current sensor.

Other objects, features, and advantages of the present invention will become apparent to those skilled in the art from the following detailed description and accompanying drawings. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the present invention, are given by way of illustration and not limitation. Many modifications and changes within the scope of the present invention may be made without departing from the spirit thereof, and the invention includes all such modifications.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred exemplary embodiment of the invention is illustrated in the accompanying drawings in which like reference numerals represent like parts throughout, and in which:

FIG. 1A is a perspective view of a motor having a rotor-embedded sensor in accordance with an embodiment of the present invention, and FIG. 1B is a side view of a rotor used in the motor illustrated in FIG. 1A;

FIG. 2 is a block diagram of a motor system that includes the motor illustrated in FIG. 1A;

FIG. 3 is a sensor lamination having a current transformer-type current sensor embedded therein in accordance with an embodiment of the present invention;

FIG. 4 is a sensor lamination having a current sensor embedded therein that detects zero-crossings of the rotor current in accordance with an embodiment of the present invention;

FIG. 5 is a sensor lamination having a Hall effect-type current sensor embedded therein in accordance with an embodiment of the present invention;

FIG. 6 is sensor lamination having a Hall effect sensor embedded therein that measures rotor flux in accordance with an embodiment of the present invention; and

FIG. 7 is a sensor lamination having a sensor embedded therein that is formed of multiple current transformers each of which is associated with a respective rotor bar to detect rotor bar fatigue in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIGS. 1A-1B, an electric motor 10 in accordance with an embodiment of the present invention is illustrated. The motor 10 comprises a stator 12 and a rotor 16 including a shaft 18. The rotor 16 and the shaft 18 are capable of rotating relative to the stator 12. As described in greater detail below, the rotor 16 comprises a rotor core 19 that is constructed using a plurality of laminations 20 including a sensor lamination 22.

Referring now also to FIG. 2, a block diagram of a motor system 30 is illustrated that includes the motor 10 of FIG. 1. As shown therein, the sensor lamination 22 includes a sensor 34 and a transmitter 36. The sensor 34 senses one or more motor operating conditions 38. Using the transmitter 36, the sensor 34 transmits information pertaining to the motor operating conditions 38 to a receiver 40 disposed on the stator 12.

The transmitter 36 and the receiver 40 preferably establish either an optical communication link or a radio frequency (RF) communication link. In particular, if an optical communication link is established, then the transmitter 36 preferably comprises a light emitting diode that produces light having an intensity which is proportional to the voltage applied to the light emitting diode, and the receiver preferably comprises an array of photodetectors that are disposed around the perimeter of the stator adjacent the path of the rotating light emitting diode. If an RF communication link is established, then the transmitter comprises an RF transmitter and the receiver comprises an RF receiver. In either case, however, a current transformer is preferably used to power the transmitter 36 using the current flowing in one of the rotor windings as a power supply so that a separate power supply is not required.

The path of the transmitter 36 during rotation of the rotor 16 is preferably in close proximity to the receiver 40. This may be achieved by providing the lamination 22 as an end lamination disposed at an extreme end of the stack of laminations 20 and 22, and by mounting the receiver 40 in an end bell of the stator 12. This arrangement also facilitates placement of the sensor 34 within the rotor core 19, as described below.

The receiver 40 is coupled to a motor control system 42. The motor control system 42 implements closed-loop feedback control and includes a current control loop which controls the current applied to the stator windings of the stator 12, or a voltage control loop which controls the voltage applied to the stator windings. The control loop (velocity or current) controls excitation of the stator windings and therefore, by induction, the rotor windings. The motor control system 42 may also include one or more additional feedback control loops such as a torque control loop, a velocity control loop and/or a position control loop. Alternatively, an open-loop control system may be used.

Depending on the condition that is sensed, the motor control system 42 may utilize the information to control the motor 10 or may instead simply communicate the information to an operator of the motor system 30. For example, if the sensor 34 is a current sensor that measures rotor winding current to provide an indication of torque and/or shaft velocity, then this information may be used as torque or velocity feedback by the control system 42 to adjust the motor operating conditions. If, on the other hand, the sensor 34 is a rotor bar fatigue sensor, then the motor control system 42 communicates the information to an operator of the motor system 30, for example, so that the operator may choose to shut down the motor system 30 and repair or replace the motor 10 when multiple rotor bars have broken.

Referring now also to FIGS. 3-9, a number of different specific embodiments (22 a, 22 b, 22 c, 22 d and 22 e) of the sensor lamination 22 are illustrated. The different laminations 22 a, 22 b, 22 c, 22 d and 22 e each have a different respective specific embodiment (34 a, 34 b, 34 c, 34 d and 34 e) of the sensor 34 embedded therein.

Referring first to FIG. 3, a sensor lamination 22 a is illustrated that contains a sensor 34 a which measures the current through one of the rotor windings. In many respects, the sensor lamination 22 a and the other laminations 20 are similarly constructed. In particular, the sensor lamination 22 a is disk-shaped and has a plurality of apertures formed therein including apertures 50, 52 and 54. The apertures 50 are preferably identically shaped, are evenly spaced around the perimeter of the lamination, a and are equidistant from the outside perimeter of the lamination . The total number of apertures 50 and the particular shape of the apertures 50 will vary from motor to motor. In this regard, it may also be noted that the relative sizes of all of the features shown in FIG. 3 will vary from motor to motor, since the sizes of some of the features are dependent on motor size and others are not.

During fabrication of the rotor 16, when the laminations 20 and the lamination 22 a are stacked one on top of another, the apertures 50 of neighboring ones of the laminations a re substantially concentrically aligned to form a plurality of slots that extend longitudinally through the rotor core 19. (In practice, a slight skew is often intentionally introduced so that the rotor slots are skewed by one slot over the length of the rotor core 19.) The laminations are then clamped together with a die that covers the region within the dashed line 56, and molten metal (e.g., aluminum) is injected into the slots formed by the apertures 50. When the molten metal cools, it forms a plurality of rotor bars 58 as well as two end rings 59 (see FIG. 1B). The rotor bars 58 extend longitudinally between the two end rings 59 and are disposed in the slots formed by the apertures in the laminations 20 and 22 a. The rotor bars 58 in combination with the end rings 59 form the windings of the rotor 16.

The aperture 52 provides an opening for the shaft 18 to fit through the rotor core 19. The aperture 52 is not filled with molten metal because it is within the region 56 that is covered by the die when the molten metal is injected into the apertures 50.

The apertures 54 are for ventilation and promote heat dissipation. Again, the apertures 54 are not filled with molten metal because they are within the region 56 that is covered by the die when the molten metal is injected into the apertures 50.

Unlike the laminations 20, the sensor lamination 22 a includes a sensor 34 a that measures the current through a rotor bar 58′ which is one of the rotor bars 58. To this end, the sensor lamination 22 a includes an additional aperture 60 that forms a cavity in the lamination 22 a and therefore in the stack of laminations 20 and 22 a as a whole. The sensor 34 a and the transmitter 36 are both contained (at least partially) within the cavity 60. The sensor 34 a is implemented in the form of a current transformer in which the rotor bar 58′ partially forms the primary winding and a multi-turn winding 62 forms the secondary winding. The remainder of the primary winding is formed by the end rings 59 and the remaining rotor winding structure, which connect the two longitudinal ends of the rotor bar 58′. Although only two turns are shown, the secondary winding 62 is in practice formed of many turns. The sensor 34 a also comprises sensing circuitry 64 that measures a voltage developed across a burden resistor 65 to provide a measurement of the current flowing in the rotor bar 58′.

During fabrication of the rotor 16, the winding 62 is inserted to the sensor lamination 22 a when the sensor lamination is first fabricated, and the sensing circuitry 64 and the transmitter 36 are added after the rotor windings have been fabricated. Thus, when the molten metal is injected into the slots 50, the sensing circuitry 64 and the transmitter 36 are not in the cavity 60. This prevents the extreme heat of this portion of the rotor fabrication process from destroying the sensing circuitry 64 and the transmitter 36. Since the winding 62 is formed only of wire, the winding 62 is able to withstand the extreme heat.

Once the molten metal has cooled and the rotor windings have formed, the die that covers the region 56 is removed and the cavity 60 is exposed. Since the lamination 22 a is an end lamination, the cavity 60 is readily accessible. If it is desirable to locate the lamination 22 a more towards the center of the stack of laminations, then the laminations 20 between the lamination 22 a and the end of the stack should be provided with similarly located apertures to permit access to the cavity 60 from the exterior of the rotor core 19.

The lamination 22 a is fabricated such that the leads of the secondary winding 62 are exposed within the cavity 60. Thus, once the molten metal has cooled, the leads of the secondary winding 62 may be easily connected to the sensing circuitry 64. The sensing circuitry 64 and the transmitter 36 are then permanently mounted within the cavity 60. Although the other laminations 20 preferably have a thickness on the order of about 0.025″, the thickness of the lamination 22 a may be made larger if necessary so that adequate space exists for the transmitter 36 and the sensing circuitry 64.

In operation, the current flowing in the rotor bar 58′ induces a current in the secondary winding 62. The flow of current in the secondary winding 62 causes a voltage to develop across the burden resistor 65. The sensing circuitry 62 measures the voltage developed across the burden resistor 65 to obtain a measurement of the current in the rotor bar 58′. The sensing circuitry 64 provides this measurement to the transmitter 36, which then transmits the measurement to the receiver 40. The measurement is then transmitted to the motor control system 42, which utilizes the measurement during normal operating conditions to control the motor 10 in closed loop fashion. For example, the magnitude of the current may be used as an indication of torque if the motor control system 42 implements torque control. Alternatively, the frequency of the current may be used as an indication of rotor velocity if the motor control system 42 implements velocity control. In either case, it is necessary to synchronize the phase of the voltage or current that is applied to the stator windings with the angular position of the rotor 16. Therefore, the phase of the current may be used by the motor control system 42 to this end.

It may be noted that the rotor 16 could also be fabricated using pre-formed rotor bars. In particular, rather than injecting molten metal into the slots defined by the apertures 50, solid metal bars may be mechanically driven into the slots (e.g., pounded in with a hammer). The advantage of this approach is that it eliminates the need for special provision to be made that allows components to be added later, since there is no concern regarding components surviving the extreme temperature of molten metal. The disadvantage of this approach is that it is more labor intensive in practice.

Referring next to FIG. 4, a sensor lamination 22 b having a current sensor 34 b embedded therein in accordance with an embodiment of the present invention. Only a portion of the sensor lamination 22 b is shown since the sensor lamination 22 b is the same as the sensor lamination 22 a except as illustrated. In this regard, it may be noted that a rotor 16 comprising the sensor lamination 22 b may be fabricated in the manner described above in conjunction with the sensor lamination 22 a.

The current sensor 34 b is used to detect the phase of the current through the rotor bar 58′. To this end, the sensor 34 b is again implemented in the form of a current transformer in which the rotor bar 58′ partially forms the primary winding and a multi-turn winding 62 forms the secondary winding. The sensor 34 b also comprises sensing circuitry 74 that includes a rectifier 75. The transmitter 36 includes a light emitting diode (LED) 76.

In operation, the current flowing in the rotor bar 58′ induces a current in the secondary winding 62. The current in the secondary winding 62 causes a voltage to develop across the rectifier 75. The output of the rectifier drives the LED 76. Accordingly, the LED 76 emits light whenever the magnitude of the voltage that is developed across the secondary winding is above zero (regardless whether the voltage is positive or negative), and does not emit light when the voltage across the secondary winding passes through zero. By using the receiver 40 to detect instances at which the LED 76 momentarily turns off, the motor control system 42 is able to detect zero-crossings of the current in the rotor bar 58′ and thereby determine its phase. The phase of the current in the rotor bar 58′ is then used by the motor control system 42 to synchronize the phase of the voltage or current that is applied to the stator windings with the angular position of the rotor 16. Additionally, the zero crossings could also be used to determine the frequency of the current in the rotor bar 58′, and thereby to determine the velocity of the rotor shaft 18.

In practice, the turn-on voltage of the LED 76 ensures that the LED 76 does not turn on immediately after passing through zero, and thus remains off for a sufficient period of time to be detected by the motor control system 42. If desired, more complicated circuitry could be utilized to detect when the current through the rotor bar 58′ passes through a predetermined level that is different than zero.

As compared to the sensor 34 a shown in FIG. 3, the sensor 34 b shown in FIG. 4 only provides information pertaining to the phase and frequency of the rotor current, and not its magnitude. However, the sensor 34 b is less complex. Likewise, the transmitter 36 also is less complex and requires less power because less information is communicated across the rotor airgap.

Referring next to FIG. 5, a portion of a sensor lamination 22 c is illustrated which contains a sensor 34 c. The sensor 34 c is a current sensor used to measure current through one of the rotor bars 58′, and comprises a magnetic core 80, a Hall effect device 82, and a biasing and sensing circuit 84. The magnetic core 80 substantially encircles the rotor bar 58′. The magnetic core 80 does not entirely encircle the rotor bar 58′ because the Hall effect device 82 is interposed between first and second adjacent ends 85 and 86 of the magnetic core 80.

In operation, the biasing and sensing circuit 84 biases the Hall effect device 80. The Hall effect device 80 is operative to detect magnetic flux produced by the current flowing through the rotor bar 58′ and thereby to obtain a measurement of the current flowing through the rotor bar 58′. The biasing and sensing circuit 84 senses the output of the Hall effect device 80 and communicates with the transmitter 36 to enable the transmitter 36 to transmit a current measurement to the receiver 40.

The current measurement obtained by the current sensor 34 c may be used in the same manner as described above in conjunction with the current sensor 34 a. The advantage of the current sensor 34 c is that it is able to measure currents having a frequency that approaches zero (i.e., direct current). Thus, the current sensor 34 c may be used at very low speeds when the slip frequency is of particular interest.

Referring next to FIG. 6, a sensor lamination 22 d is illustrated which contains a sensor 34 d used to measure magnetic flux flowing in the rotor 16. The construction and operation of the sensor 34 d is generally similar to the construction and operation of the sensor 34 c. Unlike the sensor 34 c, however, the sensor 34 d does not utilize a magnetic core. Additionally, the sensor 34 d comprises a Hall effect device 82′ that is disposed generally in between two rotor bars 58′.

In operation, the Hall effect device 82′ senses magnetic flux that flows between the two rotor bars 58′ and that is perpendicularly oriented relative to the longitudinal orientation of the rotor bars 58′. The information obtained by the sensor 34 d may be used by the motor control system 42 to synchronize the phase of the voltage or current that is applied to the stator windings with the angular position of the rotor 16.

Referring next to FIG. 7, a sensor lamination 22 e is illustrated which contains a sensor 34 e that is used to sense rotor bar fatigue (e.g., broken rotor bars). The sensor 34 e comprises a plurality of current transformers each of which comprises a secondary windings 62 wound around a respective one of the rotor bars 58. The secondary windings 62 are each terminated at a burden resistor, so that the current flowing in the rotor bar may be measured in the manner previously described in conjunction with FIG. 3. The sensor 34 e also comprises a processor circuit 90 and a multiplexer 92 used by the processor circuit 90 to sequentially test the rotor bars 58 one at a time. There is no need to examine the rotor bars 58 all at once since rotor bar fatigue is not a rapidly changing condition.

In operation, the processor circuit 90 compares the current flowing through each of the rotor bars 58 with the current flowing through the remaining rotor bars 58. If the current flowing through one of the rotor bars 58 is markedly different than the current flowing through the remaining rotor bars 58, then this indicates a failing rotor bar. Of course, if no current is flowing through one of the rotor bars 58, then this indicates that the particular rotor bar is broken. The transmitter 36 then communicates whether and how many rotor bars are failing or broken to the receiver 40. Alternatively, the transmitter 36 could transmit current measurements from each of the secondary windings 62 to the motor control system 42, and the motor control system 42 could then perform the comparisons of the current measurements.

The sensor 34 e facilitates predictive maintenance of the motor 10. The motor control system 42 knows how many rotor bars are failing or broken and, on this basis, a prediction can be made of the remaining useful life of the motor 10. The motor 10 may therefore be replaced during off hours in advance of failure, rather than during the middle of a production run, for example.

In addition to being used to measure the current in the rotor windings, the present invention could also be used in a number of other ways. For example, a sensor lamination may be constructed that contains a sensor that monitors the temperature of the rotor 16. In such an arrangement, the sensor 34 may be provided in the form of one or more resistive temperature devices (RTDs), which have a resistance that varies according to sensed temperature. If multiple RTDs are used, a multiplexer can select between the RTDs so that the transmitter 36 transmits temperature information from a selected one of the RTDs to the receiver 40. The temperature information may then be used by the motor control system 42 to detect when the rotor 16 (and therefore the motor 10) is overloaded, for example.

In another embodiment, a sensor lamination could be provided that contains a sensor that measures rotor air gap. In such an arrangement, the sensor lamination may comprise a proximity sensor disposed at an outer periphery of the sensor lamination. By monitoring the distance between the location on the rotor at which the proximity sensor is placed and the inner periphery of the stator, variations in the distance between the inner periphery of the stator 12 and the outer periphery of the rotor 16 may be detected. Such variations may be caused, for example, by stator eccentricity.

In still yet another embodiment, a sensor lamination may be constructed that measures rotor torque. In such an arrangement, the sensor lamination may have a section that is formed of piezoelectric material and that extends from the center of the lamination to the outer periphery. In operation, torsion of the sensor lamination would place pressure on either side of the piezoelectric material, causing the piezoelectric material to produce a voltage indicative of motor torque.

It is therefore seen that the present invention advantageously provides a way of sensing rotor-associated conditions. These conditions include conditions that relate to the general state of the rotor (e.g., whether one or more rotor bars is broken or otherwise overly fatigued, whether the rotor and therefore the motor is overloaded, and so on). These conditions also include conditions that are parameters which can be measured and assigned a value (e.g., the magnitude, frequency and phase of the current flowing in one of the rotor bars). Although numerous embodiments have already been described that are capable of sensing various different conditions, it will appreciated that numerous other embodiments are also possible.

It should also be noted that the different embodiments are not mutually exclusive. For example, a sensor lamination could be provided that includes one or more of the embodiments of the invention described above. Also, the present invention can be used in conjunction with any electromechanical device having a rotor that is capable of rotating relative to a stator. For example, the present invention may also be used in conjunction with electric generators.

Many other changes and modifications may be made to the present invention without departing from the spirit thereof. The scope of these and other changes will become apparent from the appended claims. 

What is claimed is:
 1. An induction motor control system comprising: (A) an induction motor comprising (1) a stator, and (2) a brushless rotor having a winding, said rotor being capable of rotating relative to said stator, and said rotor having a sensor embedded therein, said sensor being capable of obtaining information pertaining to excitation of said winding; and (B) a motor control system, said motor control system including a feedback control loop, said feedback control loop receiving said information pertaining to said excitation of said winding as a feedback parameter and controlling further excitation of said winding during normal operating conditions at least partially in accordance with said feedback parameter obtained by said rotor-embedded sensor.
 2. A motor control system according to claim 1, wherein said rotor comprises a rotor core, said rotor core including a plurality of laminations that are stacked one on top of another, and wherein said sensor is disposed within said plurality of laminations.
 3. A motor control system according to claim 2, wherein one of said plurality of laminations is a sensor lamination, and wherein said sensor is at least partially disposed within a cavity formed in said sensor lamination.
 4. A motor control system according to claim 1, wherein said rotor comprises a plurality of rotor windings one of which is said winding, said plurality of rotor windings comprising a plurality of rotor bars, and wherein said sensor is a current sensor, said current sensor measuring current through at least one of said plurality of rotor bars.
 5. A motor control system according to claim 1, wherein said sensor is a current sensor.
 6. A motor control system according to claim 1, wherein said sensor is a flux sensor.
 7. An induction motor control system comprising: (A) an induction motor comprising (1) a stator; and (2) a brushless rotor, said rotor being capable of rotating relative to said stator, said rotor including (a) a rotor core, said rotor core including a plurality of laminations, each of said plurality of laminations having a plurality of apertures formed therein, said plurality of laminations being stacked one on top of another such that apertures of neighboring ones of said laminations are at least substantially concentrically aligned to form a plurality of slots that extend longitudinally through said rotor core, (b) a plurality of rotor windings, said plurality of rotor windings comprising first and second end rings and a plurality of rotor bars, said first and second end rings being disposed at opposite ends of said rotor core, and each of said plurality of rotor bars being disposed between said first and second end rings in a respective one of said slots formed by said apertures in said plurality of laminations, and (c) a sensor, said sensor being disposed within said plurality of laminations, and said sensor being capable of obtaining information pertaining to excitation of at least one of said plurality of rotor windings; and (B) a motor control system, said motor control system including a feedback control loop, said feedback control loop receiving said information pertaining to said excitation of said at least one winding as a feedback parameter and controlling further excitation of said winding during normal operating conditions at least partially in accordance with said feedback parameter obtained by said rotor-embedded sensor.
 8. A motor control system according to claim 7, wherein said sensor is a current sensor, said current sensor measuring current through said at least one winding.
 9. A motor control system according to claim 7, wherein said sensor is a current sensor, wherein said current sensor comprises a current transformer, said current transformer having a primary winding that is partially formed by one of said rotor bars, said one of said rotor bars forming at least a portion of said at least one winding, and wherein said current sensor measures current through said one of said plurality of rotor bars.
 10. A motor control system according to claim 7, wherein said sensor is a current sensor, wherein said current sensor comprises a crossing detector that changes state when a current through one of said rotor bars passes through a predetermined level, said one of said rotor bars forming at least a portion of said at least one winding.
 11. A motor control system according to claim 7, wherein said sensor is a current sensor, and wherein said current sensor comprises a magnetic core and a Hall effect device, said magnetic core substantially encircling one of said rotor bars, said one of said rotor bars forming at least a portion of said at least one winding, and said Hall effect device being interposed between first and second adjacent ends of said magnetic core.
 12. A motor control system according to claim 7, wherein said sensor is a flux sensor, said flux sensor sensing magnetic flux that flows in said rotor core.
 13. A motor control system according to claim 7, wherein said plurality of rotor bars are formed of metal, said metal having been injected into said plurality of slots while in a molten state, and wherein said cavity is disposed at an end of said rotor core, said cavity being formed by the failure of said metal to flow into an aperture formed in at least one of said laminations while in said molten state.
 14. A motor control system according to claim 7, wherein said rotor further comprises a transmitter and said stator further comprises a receiver, said transmitter and said receiver cooperating to establish a communication link between said rotor and said stator, said communication link permitting information pertaining to said motor operating condition to be communicated from said rotor to said stator.
 15. A motor control system according to claim 7, wherein said plurality of laminations include a sensor lamination having a cavity formed therein, and wherein said sensor is at least partially disposed within said cavity formed in said sensor lamination.
 16. A method of controlling an induction motor having a stator and a brushless rotor that rotates relative to said stator, said rotor including a winding and a sensor, the method comprising: sensing excitation of said winding of said induction motor using said sensor; transmitting information pertaining to said excitation of said winding from said brushless rotor to a motor control system having a feedback control loop; receiving said information pertaining to said excitation of said at least one winding, said information being received as a feedback parameter for said feedback control loop; and controlling further excitation of said winding during normal operating conditions using said motor control system, said further excitation of said winding being controlled at least partially in accordance with said feedback parameter obtained by said rotor-embedded sensor.
 17. A method according to claim 16 wherein, during said sensing step, said sensor senses current flowing through a rotor bar, said rotor bar partially forming a rotor winding of said rotor.
 18. A method according to claim 16, wherein said controlling step includes using said information to synchronize a phase of an excitation signal applied to a winding of said stator with an angular position of said rotor.
 19. An electric motor system comprising: (A) an induction motor including (1) a stator; and (2) a rotor, said rotor being capable of rotating relative to said stator, said rotor including (a) a rotor core, said rotor core including a plurality of laminations, each of said laminations having a plurality of apertures formed therein, said plurality of laminations being stacked one on top of another such that apertures of neighboring ones of said laminations are at least substantially concentrically aligned to form a plurality of slots that extend longitudinally through said rotor, and said plurality of laminations including a sensor lamination, said sensor lamination having a cavity formed therein, (b) a plurality of rotor windings, said plurality of rotor windings comprising first and second end rings and a plurality of rotor bars, said first and second end rings being disposed at opposite ends of said rotor core, and each of said plurality of rotor bars being disposed between said first and second end rings in a respective one of said slots formed by said apertures in said plurality of laminations, and (c) a current sensor, said current sensor being at least partially disposed within said cavity formed in said sensor lamination, and said sensor obtaining a current measurement, said current measurement pertaining to the flow of current through at least one of said rotor bars; and (B) a motor control system, said motor control system receiving said current measurement from said sensor and using said current measurement to control said motor.
 20. An electric motor system according to claim 19, wherein said motor control system includes a feedback control loop, and wherein said current measurement is used to derive an input for said feedback control loop. 